In general, a conventional bone conduction speaker has a construction shown in each of FIGS. 6 and 7. The conventional bone conduction speaker shown in FIG. 6 is constructed of a yoke 23, which is provided with a voice coil 21 and a magnet 22, wherein: through a diaphragm or vibrating plate 25 fixedly mounted on an upper surface of the yoke 23 and a vibrating block 27, the yoke 23 is mounted on a ceiling surface of the interior of a housing 24 in a suspending manner.
The above construction of the conventional bone conduction speaker is advantageous in that it is easily assembled and capable of obtaining a relatively large output. On the other hand, the conventional bone conduction speaker is disadvantageous in that: since the yoke 23 is suspended from the vibrating plate 25, a value of “Q” (a sharpness of resonance, which is directly proportional to an equivalent mass of the vibrating system) of a resonance portion which is determined by a mass of a vibrating system and the like increases to have a curve of the frequency characteristics of the bone conduction speaker be out of a flat shape. As a result, a reproduced sound having a minimum resonance frequency becomes loud to impair the sound in tone quality, which the user often feels as an unfavorable vibration. Further, due to the above construction, when the sound varies in frequency, an abnormal resonance of the vibrating plate tends to occur, which leads to an abnormal sound reproduced at a frequency of the resonance. This is a problem inherent in the conventional bone conduction speaker.
On the other hand, the other one (shown in FIG. 7) of the conventional bone conduction speakers has a construction in which: the yoke 23 provided with the voice coil 21 and the magnet 22 is fixedly mounted on a bottom surface of the interior of the housing 24 which is provided with an opening 28 in its upper surface; the vibrating plate 25 is fixedly mounted on an upper surface of the yoke 23; and, a vibrating portion 26 is vibrated in operation, fixedly mounted on the vibrating plate 25 through the vibration block 27, and exposed to the outside through the opening 28 of the housing 24, wherein the vibrating portion 26 is held in abutting contact with a head portion of a user in use.
Also in this case, as is in the above-described bone conduction speaker, since the vibrating system is suspended, there is the same problem as that described in the above more or less.
Further, in the case of this construction, since an air gap or opening 28 is disposed adjacent to an outer peripheral portion of the vibrating portion 26, there is a fear that some foreign material such as dust, the user's sweat and like foreign materials enters the housing 24, which often causes a failure of the conventional bone conduction speaker.
Since the conventional bone conduction speaker suffers from the above-mentioned problems, it is an object of the present invention to provide a bone conduction speaker free from the above problems. In other words, it is an object of the present invention to provide a bone conduction speaker having a construction in which: an unnecessary vibration of a vibrating system is suppressed so that any abnormal sound resulted from an abnormal resonance is prevented from being issued, whereby the sound issued from the bone conduction speaker is sufficiently improved in tone quality.
It is another object of the present invention to provide a bone conduction speaker, which is sufficiently tough so as to not break even when it is excessively vibrated and dropped from the user's head and therefore subjected to a mechanical shock.